Process of continuously producing polyesters or copolyesters

ABSTRACT

Process for producing polyesters and copolyesters, wherein a continuous esterification of dicarboxylic esters and diols or a continuous transesterification of dicarboxylic esters with diols is effected in “n” reaction pressure stages connected in series, the pressure in the first reaction pressure stage is higher than that of the remaining reaction pressure stages, and successively decreases from reaction pressure stage to succeeding reaction pressure stage, the combined vapor flows of the individual reaction pressure stages are rectified in a rectifying column, and the diol component obtained at the bottom of the rectifying column is recirculated at least to the first reaction pressure stage, and wherein at least the last reaction pressure stage operates at a vacuum, the pressure in the rectifying column is greater than that of the last reaction pressure stage, and the vapors discharged from the last reaction pressure stage are condensed to the pressure existing in the rectifying column.

This is a 371 of PCT/EP01/13776 filed 27 Nov. 2001 (International filingdate).

This invention relates to a process of producing polyesters orcopolyesters by continuous esterification of dicarboxylic esters anddiols or by continuous transesterification of dicarboxylic esters withdiols in n reaction pressure stages connected in series, wherein thepressure existing in the first reaction pressure stage, which is higheras compared to the other reaction pressure stages, successivelydecreases from reaction pressure stage to reaction pressure stage, thecombined vapor flows of the individual reaction pressure stages areintroduced into a rectifying column and rectified, and the diolcomponent obtained at the bottom of the rectifying column isrecirculated at least to the first reaction pressure stage.

BACKGROUND OF THE INVENTION

It is known that the continuous production of polyethylene terephthalate(PET) from terephthalic acid (TPA) and ethane diol (EG) is effected byesterification in a first reaction stage under an elevated pressure andin a further reaction stage under normal pressure. To furtheroligomerize the esterification products, there are subsequently providedtwo prepolycondensation stages operating under a vacuum and a finalreactor for producing the finished polyester melt in a fine vacuum.

For performing such 5-stage process, EP-B-0 244 546 describesapparatuses for vapor condensation and vacuum generation by means ofmultistage steam jet pumps each including a preceding spray condenserand succeeding direct-contact condensers. The steam jet pumps areoperated with purified process water vapor from the rectifying columnabove the first reaction pressure stage, which process water vapor hasan excess pressure of 2 to 3 bar, a recirculation of cooling water beingeffected from the direct-contact condensers towards the rectifyingcolumn. The disadvantages of this process substantially consist in ahigh organic contamination of the waste water in open process circuitsand in a large and cost-intensive amount of apparatus on the whole, aseach reaction stage, also the second reaction stage of theesterification, has a separate condensation system. Moreover, theformation of diethylene glycol (DEG) is increased as a result of theelevated pressure in the first reaction pressure stage and at the bottomof the rectifying column.

In general it should be noted that when rectifying the originalsuperheated vapor stream with a weight content of about 50% EG in therectifying column, a considerable part of the steam gets lost bothmaterially and energetically as operating steam for operating steam jetpumps.

An improvement of the above-described process is achieved in that thevapors of the first and second reaction pressure stages of theesterification are supplied together to a rectifying column, and twovacuum stages are combined in a single prepolycondensation stage.

The disadvantage of a single prepolycondensation stage, however, is asize-related limitation of the capacity as a result of an increasedgeneration of gas and the increasing risk of an entrainment of dropletsas well as a generally undesired increase of the content of carboxylicgroups in the polyester or copolyester as a result of a fasterevaporation of EG during the prepolycondensation without pressuredifferentiation.

The subject-matter of U.S. Pat. No. 4,670,580 is a 4-stage total processof producing PET, wherein in the second reaction stage an esterificationunder a vacuum is provided, and wherein a typical acid conversion of 97%is effected. This conversion appears to be too low for a lasting gasrelief in the pre-polycondensation stage, i.e. an increased conversionrequires longer dwell times, i.e. larger and more expensive reactionapparatuses. In any case, an additional rectifying column withrecirculation of EG or some other, separate condensation system withsubsequent recovery of EG is necessary.

In the esterification under a vacuum in the second reaction stage, thereare also considerable reservations as regards the quality andflexibility of the plant with respect to the addition of additives; aswithout normal pressure and without stirring the melt, in particular inthe textile PET production, admixing additives free from agglomerate andwithout evaporation losses is quite difficult.

It is the object of the present invention to improve the above-describedprocess such that the amount of apparatus required and the operatingcosts for cooling water and for energy are decreased without impairingthe quality of the polyesters or copolyesters produced and theflexibility of the process.

SUMMARY OF THE INVENTION

This object is solved in that proceeding from the above-describedprocess at least the last reaction pressure stage is operated atsubatmospheric pressure of 100 to 900 mbar (absolute), the pressure inthe rectifying column is larger than in the last reaction pressurestage, and the vapors discharged from the last reaction pressure stageare condensed to the pressure existing in the rectifying column.

DETAILED DESCRIPTION

The various processes for esterification or transesterification areperformed in n reaction pressure stages connected in series, thepressure decreasing successively with increasing monomer conversions oradvancing sequence of reaction pressure stages.

In the esterification process for producing polybutylene terepththalate(PBT), the reaction pressure stages are exclusively operated under avacuum, whereas in the esterification process for producing PET andpolytrimethyl terephthalate (PTT) the first reaction pressure stage isoperated under an excess pressure and the last reaction pressure stageis operated under a vacuum. In the transesterification process,proceeding from dimethyl terephthalate (DMT) and 1,4-butanediol (BDO),the first reaction pressure stage is substantially operated under normalpressure.

For the technical effect of the inventive process it is irrelevantwhether the individual reaction pressure stages are disposed in a singleapparatus or constitute separate reactors.

For performing the inventive process, at least two reaction pressurestages are required. For a particularly advantageous procedure it isexpedient to utilize three reaction pressure stages. The application ofa process with four reaction pressure stages likewise is possible,wherein the vapors of the last reaction pressure stage are condensed tothe pressure of the penultimate reaction pressure stage or the pressureof the rectifying column.

A condensation of the vapors of the last reaction pressure stage and acorrespondingly larger rectifying column are avoided by condensing thevapors to a comparatively higher pressure. For the case that the processis performed by using two reaction pressure stages, the height of thepressure in the rectifying column lies between the pressure in the firstreaction pressure stage and that of the second reaction pressure stage.When using more than two reaction pressure stages, the pressure in therectifying column is not larger than the pressure existing in thepenultimate reaction pressure stage.

The condensation of the vapors is effected by means of a compressor,blower or ventilator. However, it is particularly advantageous toperform the condensation of the vapors by means of a gas jet pump byusing superheated vapors from the first reaction pressure stage asoperating steam, since vapors of the first reaction pressure stage areavailable at no cost in a sufficient quantity and thus the condensationof vapors by using compressors, blowers or ventilators is onlyconsidered in the second place.

A particular aspect of the inventive process should be seen in that thepressure existing in the rectifying column can be controlled by anexhaustion of residual gas, which is effected behind the condensermounted at the head of the rectifying column, for instance by means of aliquid jet pump, a liquid ring pump or also by means of a blower.

Especially in PET esterification, a further aspect of the inventiveprocess is the pressure control in the vacuum esterification stage underpartial load in consideration of a pressure decreasing with smaller flowrate, for instance from about 1750 mbar at 100% flow rate to about 1250mbar at 50% flow rate, with constant filling level in the first reactionpressure stage and a still low formation of DEG. As a result of thereduced operating steam pressure at the gas jet pump under partial load,the pressure of the vacuum stage is increased from about 500 mbar underfull load to about 800 mbar with 50% flow rate, each with constantcounter-pressure of about 1000 mbar at the rectifying column.

For quality reasons, however, a constant vacuum in the vacuumesterification stage is required, independent of the flow rate.

In accordance with the invention, this constancy of the pressure isachieved without larger dimensions of the rectifying column with acontrolled decrease of the pressure in the rectifying column to 640 to700 mbar at a flow rate of 50%.

The corresponding for vacuum of the rectifying column is generated andcontrolled via an air exhauster disposed behind the column headcondenser, alternatively via a liquid jet pump or a liquid ring pump.

For continuously producing PET esterification product, there canpreferably be used a three-stage process with the parameters indicatedin Table 1 by way of example.

TABLE 1 Stages No. I II III Temperature (° C.) 243–269° 255–270°262–272° Pressure (bar) 1.2–2.5  1.0–1.1  0.3–0.6  Dwell time (min)120′–450′   35′–110′   45′–130′ 

To the first reaction pressure stage (I) operated under excess pressure,EG and TPA are simultaneously introduced as paste in a molar ratio <2,preferably ≦1.2, whereby TPA conversions of 88 to 93% are achieved. Thesecond reaction pressure stage (II) is operated under normal pressure,and a conversion of 94 to 97.4%, preferably 95.5 to 96.7% is achieved.The reaction pressure stage (II) is used for admixing additives and foradjusting the molar EG content in the end product. In the third reactionstage (III) operated under a vacuum, the conversion is increased to >97to 99%, preferably to 97.9 to 98.5%.

By means of a gas jet pump, the vapors from the third reaction pressurestage (III) are condensed to the pressure of the rectifying column. Theoperating steam is withdrawn from the vapors of the first reactionpressure stage (I). Under nominal load, the pressure of the vapors isincreased by a factor of 1.6 to 2.6 during the condensation of thevapors from the last stage.

The continuous production of PTT esterification product is expedientlyeffected by means of a two-stage process with the parameters indicatedin Table 2 by way of example, using a paste prepared from1,3-propanediol (PDO) and TPA in a molar ratio of 1.15 to 2.20,preferably ≦1.40.

TABLE 2 Stage I II Introduced 1.15–2.20 — molar ratio PDO/TPATemperature (° C.) 243–263 242–262 Pressure (bar) 1.0–3.5 0.7–1.1 Dwelltime (min) 140–360 30–80

When producing PBT transesterification product by means of a two-stageprocess, butanediol (BDO) and liquid DMT are separately introduced intothe first reaction pressure stage with a molar ratio of 1.2 to 1.4. Theprocess parameters are indicated in Table 3 by way of example.

TABLE 3 Stage I II Introduced 1.15–1.50 — molar ratio BDO/DMTTemperature (° C.) 165–185 195–210 Pressure (bar) 0.90–1.20 0.4–0.7Dwell time (min)  70–180 40–90

The inventive process is schematically represented in the drawing andwill subsequently be explained in detail by way of example. In thedrawing

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a process diagram for producingesterification/transesterification products by means of two reactionpressure stages for PPT and PBT;

FIG. 2 shows an aspect of the process diagram in accordance with FIG. 1;and

FIG. 3 shows a process diagram for producing PET esterification productby means of three reaction stages.

As shown in FIG. 1, a prefabricated diol/dicarboxylic acid paste isintroduced into the first reactor (2) via line (1). The product streamdischarged from the first reactor (2) via line (3) is supplied to thesecond reactor (4). Via line (5), the finished esterification product isdischarged from the second reactor (4). The vapors discharged from thefirst reactor (2), minus a small bypass stream conducted via line (7)and provided for pressure regulation in the first reaction stage, aresupplied to a heated gas jet pump (8) via line (6), by means of whichgas jet pump the gas stream introduced via line (9), which is composedof the vapors of the second reactor (4) discharged via line (10) and aballast stream to be used for controlling the pressure in the secondreactor (4) and flowing through line (11), is condensed to the pressureof the rectifying column (12). The vapors discharged from the firstreactor via line (6) and the vapors discharged from the second reactorvia lines (10, 9) are charged into the gas jet pump (8). In line (14),the vapor stream discharged from the gas jet pump (8) via line (13) iscombined with the vapor bypass stream supplied via line (7) and chargedinto the inlet of the rectifying column (12), in which the low-boilingcomponents are separated from the high-boiling diol component. Thelow-boiling components discharged at the head of the rectifying column(12) via line (15) are condensed in the condenser (16). The condensateflowing off via line (17) is divided into a reflux stream and a productstream via the reflux tank (18). The reflux stream is supplied to thehead of the rectifying column (12) via line (19), and the product streamis withdrawn via line (20). The diol enriched at the bottom of therectifying column (12) is for the most part supplied to the firstreactor (2) via lines (21, 22), and only a small part branched off vialine (23) is supplied to the second reactor (4). It is possible to feedadditives and/or comonomers into the second reactor (4) via line (24).In addition, an aliquot of the diol flowing through line (22) can bebranched off via line (22 a) and be used for preparing the paste.

For the case that the process diagram represented in FIG. 1 is used forthe purpose of transesterification, diol is fed into the first reactor(2) via line (1) and DMT is fed into the first reactor (2) via line(25).

To be able to operate the process diagram represented in FIG. 1 alsounder a vacuum, it is provided in accordance with FIG. 2 to effect aresidual gas compression behind the condenser (16) by means of a pump(27) exhausting the residual gas from the reflux tank (18) via line(26). The condensate flowing out of the reflux tank (18) via line (28)is brought to normal pressure by means of a dipping tank (29), and thecondensate is then removed from the process via line (30).

In the process diagram as shown in FIG. 3, EG/TPA paste is introducedinto the first reactor (32) via line (31), the outlet of said reactorbeing fed into the second reactor (34) via line (33). The product streamleaving the second reactor (34) via line (35) is supplied to the thirdreactor (36), from which the finished esterification product isdischarged via line (37). Via line (38), the vapors discharged from thefirst reactor (32), minus a small bypass stream flowing through line(39) and used for pressure control in the first reaction stage (32), aresupplied to the heated gas jet pump (40). Via line (43), an amount ofballast stream from the vapors of the second reactor (34) flowingthrough line (42) is added to the vapors discharged from the thirdreactor (36) via line (41), in order to condense the vapors of the thirdreactor (36) to the pressure of the rectifying column by means of thegas jet pump (40). If necessary, the vapors discharged from the secondreactor (34) via line (42) are combined with the vapors discharged fromthe gas jet pump (40) via line (45) and with the bypass stream suppliedvia line (39), upon flowing through the pressure maintaining valve (44),so that the pressure of the second reactor (34) is larger than or equalto the pressure in the rectifying column. Via line (46), the combinedvapors are fed into the rectifying column (47), where the low-boilingcomponents are separated from the high-boiling diol component. Thelow-boiling components discharged at the head of the rectifying column(47) via line (48) are condensed in the condenser (49). Via line (50),the condensate flows into the reflux tank (51), from which part is againcharged into the head of the rectifying column (47) via line (52). Forvacuum operation of the rectifying column (47) under partial load, thereflux tank (51) is connected with a suction pump (54) via line (53).The condensate discharged from the reflux tank (51) via line (55) isbrought to normal pressure in the dipping tank (56) and removed via line(57) for further processing. The ethane diol containing a small amountof high-boiling components at the bottom of the rectifying column (47)is recirculated for the larger part to the first reactor (32) via lines(58, 59) and for the smaller part to the second reactor (34) via line(60). From the diol flowing through line (59) an aliquot can be branchedoff via line (59 a) and be used for preparing the paste.

1. A process of producing polyesters and copolyesters by continuousesterification of dicarboxylic esters and diols or by continuoustransesterification of dicarboxylic esters with diols in n reactionpressure stages, where n=at least 2, connected in series, wherein thepressure in the first reaction pressure stage is higher than thepressure of the remaining reaction pressure stages, and the pressuresuccessively decreases from reaction pressure stage to reaction pressurestage, the combined vapor flows of the individual reaction pressurestages are introduced into a rectifying column and rectified, and thediol component obtained at the bottom of the rectifying column isrecirculated at least to the first reaction pressure stage, at least thelast reaction pressure stage in the series has a vacuum, the pressure inthe rectifying column is greater than that in the last reaction pressurestage, and vapors discharged from the last reaction pressure stage arecondensed to the pressure existing in the rectifying column.
 2. Theprocess as claimed in claim 1, wherein said reaction pressure stages are2 to 4 reaction pressure stages connected in series.
 3. The process asclaimed in claim 1, wherein, for producing a polyethylene terephthalate(PET) esterification product, n=3, a feed mixture is supplied to thefirst reaction pressure stage, which is operated at a pressure of 1.1 to5.0 bar, and is followed by the second reaction pressure stage, operatedat a pressure of 0.5 to 1.5 bar which is then followed by the thirdreaction pressure stage operated at a vacuum of 0.1 to 0.9 bar.
 4. Theprocess as claimed claim 1, wherein, for producing PTT esterificationproduct, n=2, a feed mixture is supplied to the first reaction pressurestage which is operated at a pressure of 1.0 to 5.0 bar, and is followedby a second reaction pressure stage, which is operated at a pressure of0.5 to 1.3 bar.
 5. The process as claimed in claim 1, wherein n=2 and,for producing PBT transesterification product, starting substances aresupplied to the first reaction pressure stage operated at a pressure of0.8 to 1.2 bar, which is followed by the second reaction pressure stageoperated at a pressure of 0.2 to 0.8 bar.
 6. The process as claimed inclaim 1, wherein vapors discharged from the last reaction pressure stageare condensed by means of a compressor, blower or ventilator.
 7. Theprocess as claimed in claim 1, wherein vapors discharged from the lastreaction pressure stage are condensed by means of a gas jet pump.
 8. Theprocess as claimed in claim 7, wherein vapors of the first reactionpressure stage are used as operating fluid for the gas jet pump.
 9. Theprocess as claimed in claim 1, wherein a condenser is disposed at thehead of the rectifying column, and the pressure existing in therectifying column is controlled by a residual gas exhaustion providedbehind said condenser.
 10. The process as claimed in claim 9, whereinthe residual gas exhaustion is performed by means of a liquid jet pump,a liquid ring pump or a blower.
 11. The process as claimed in claim 1,wherein the esterification products or transesterification products aresubjected to a prepolycondensation and polycondensation.
 12. The processas claimed in claim 1, wherein an aliquot of the diol recirculated tothe first reaction pressure stage is branched off and used for preparinga diol/dicarboxylic acid paste, which is introduced into the firstreaction pressure stage.